One of the unexpected answers is that some delete books after being read. Not something I've ever considered and seems odd given the amount of storage on book readers

I do that in order to avoid to browse through too many books in the library, whether using shelves or not. Oh and also to prevent battery drains or sluggishness (it might happen, maybe, when the device has over a certain amount of books).
Basically it's for the first reason And I do the same with my music player, I don't keep too many files. I prefer to rotate them.

One of the unexpected answers is that some delete books after being read. Not something I've ever considered and seems odd given the amount of storage on book readers

The books on my reader is my high priority queue of books to read. So I delete the books when I have read them. And why does it matter? I nearlyt always have network available and can re-download the book whenever I want.

I would say that in this age of never being disconnected from the net it is surprising that people want to save things locally on devices.

I have all my books backed up in calibre, and I always forget titles that I've read anyway, so I delete them from my kindle when I'm done. To me, it feels not like deleting a book or throwing it away as it feels like putting the book back on the shelf when I'm done.

That's a strange thing to do. Me, I just throw away my ebook reader and buy another one.

Tsk. Tsk. Do you realize how many trees they cut down to make an ereader? Not to mention how much money you're wasting? You could save the environment and recoup a couple of dollars by bringing it to a used book store. Just be sure to flip the book to the first page first, so that it's ready for the next reader.

I have all my books backed up in calibre, and I always forget titles that I've read anyway, so I delete them from my kindle when I'm done. To me, it feels not like deleting a book or throwing it away as it feels like putting the book back on the shelf when I'm done.

Or do what I do:

Create a column in calibre named READ and, when finished with the book, just mark 'Y'.